The present invention relates to a process for producing expandable beads of vinyl aromatic polymer resins, and more particularly to a process for producing expandable beads of vinyl aromatic polymer resins, wherein the polymer beads are impregnated with a blowing agent in the presence of a surface-active agent or emulsifier.
Expandable beads of thermoplastic resins such as polystyrene are currently used for manufacturing cellular plastic materials. These expandable beads contain a blowing or expanding agent having a boiling point lower than the softening point of the resin. When heated, the resin is thereby expanded or foamed. Generally, the beads are pre-expanded and are then deposited into a mold which is heated, for instance by introducing steam under pressure in the filled and closed mold, to further expand the beads and cause the beads to fuse together. This method can be applied for the manufacture of foamed sheets which are suitable for use as insulating materials, as packing material for electronic devices, and for producing drinking cups, and the like.
It is also known that the expandable beads which are used for the manufacture of these products should have a substantially uniform size. Moreover, the foamed products have better properties when the cells within the beads have a uniform and small size. This uniformity of the cells within the beads depends on the method which has been used for the impregnation of the beads with the blowing agent.
In the manufacture of expandable beads, the beads of vinyl aromatic polymer resin are prepared by suspension polymerization and are then impregnated with a liquid blowing agent, generally a low molecular weight aliphatic hydrocarbon, such as pentane. Conventionally, the resin beads are impregnated by direct contact with the blowing agent, typically by injecting the blowing agent into the beads while in aqueous suspension. With conventional processes for producing expandable beads, however, a uniform dispersion of the blowing agent within the beads cannot be obtained. The blowing agent is mainly located in the skin of the beads and the expandable beads have, therefore, cells whose sizes are not uniform.
In order to remedy this problem, it has been proposed to add to the suspension a surface-active agent, for instance a polyoxyethylene compound with an HLB (hydrophilic-lipophilic balance) of at least 15, in order to promote a uniform dispersion of the blowing agent within the beads. This method is disclosed in British Pat. No. 1,174,749, which teaches that surface-active agents with an HLB lower than 15 are not suitable for producing expandable beads wherein the blowing agent is uniformly dispersed. However, the use of the surface-active agents suggested in this patent has been found to have an adverse effect on the stability of the suspension, resulting in expandable beads having inferior properties and uniformity in size.